Microstructures are being used more and more in a variety of applications such as chemical processes, medical procedures, and space navigation systems. Several specific kinds of microstructures which are desirable in such applications are microvalves, micropumps, microsensors, and microfans. The development of these micromechanical structures requires comparably sized micromotors to drive these microstructures. However, the technology necessary to fabricate micromotors is in a state of infancy. A great amount of effort is presently being directed toward developing practical micromotors which can be fabricated using microfabrication techniques similar to those used in manufacturing semiconductors so that they can be economically produced on a mass scale in order to satisfy present demand. Unfortunately, the microscopic nature of these structures and motors makes the implementation of the simplest idea complex, tedious and time consuming, often resulting in a non-intuitive solution.
Various operating principles have been considered for micromotors such as electrostatic, ultrasonic, dielectric induction, and magnetic. Of the several proposed operating principles, electrostatic and magnetic drives have generally been favored.
Electrostatic micromotors operate by selectively applying a potential difference between a rotor and selected poles of a stator surrounding the rotor. As a result, the rotor poles closest to the charged stator poles of an opposite charge are pulled toward the stator pole causing the rotor to rotate. A limitation to the speed and rotational force of the electrostatic micromotor is the threshold voltage at which the electric field breaks down in the air gaps between the stator poles and rotor poles. This threshold voltage is controlled by a number of factors such as temperature, pressure and surface smoothness. Examples of electrostatic micromotors can be found in U.S. Pat. No. 5,252,881 to Muller et al., U.S. Pat. No. 4,943,750 to Howe et al., and U.S. Pat. No. 5,013,954 to Shibaike et al.
Magnetic micromotors generally fall into one of two categories, salient, meaning with poles, or non-salient, meaning without poles. The two types are most easily differentiated by the fact that variable reluctance magnetic micromotors usually include poles, while variable inductance micromotors usually do not include poles. Unlike electrostatic micromotors which are driven by voltage, magnetic micromotors are driven by magnetic flux typically generated by current flowing through an inductive component in accordance with Ampere's law. An example of a magnetic micromotor can be found in U.S. Pat. No. 5,113,100 to Taghezout.
Though not presently developed to the extent of electrostatic micromotors, magnetic micromotors have several advantages over electrostatic micromotors with regard to the environment in which the motors operate and the functions which they are capable of performing. For example, magnetic micromotors may be used in conductive fluids for bio-medical or chemical applications. Additionally, since magnetic micromotors usually require low driving voltages, the operation of magnetic micromotors is not inhibited by environments where high driving voltages are unacceptable or unattainable. Lastly, magnetic micromotors are often preferred over electrostatic micromotors because of their ability to achieve relatively high torques. Consequently, because of these advantages, magnetic micromotors are more attractive than electrostatic micromotors for many applications.
However, the development of a practical magnetic micromotor has been frustrated because of various difficulties encountered in their fabrication. Magnetic micromotors have previously been implemented by placing a permanent magnet rotor onto integrated planar coils and then introducing externally applied magnetic fields onto the assembled high-permeability moving parts, or by assembling electroplated rotors onto stators fabricated with wire-bonded coils. A reason that these approaches have been taken is due to the difficulty in fabricating three dimensional `wrapped` coils using an integrated and planar fabrication process, as well as due to the lack of techniques to fabricate the rotor and the stator in a fully integrated fashion using electroplating techniques. However, recently a new three dimensional planar integrated meander-type inductive component has been proposed and demonstrated in Chong Ahn, Yong Kim and Mark Allen, "A Planar Variable Reluctance Magnetic Micromotor With Fully Integrated Stator and Wrapped Coils," IEEE Micro Electro Mechanical Systems Workshop, pp. 1-6, Fort Lauderdale, Fla., 1993, the disclosure of which is incorporated herein by reference, in which multilevel magnetic cores were `wrapped` around planar meander conductors. This configuration can be thought of as the result of interchanging the roles of the conductor wire and magnetic core in a conventional inductor. With this integrated inductive component, it is possible to guide magnetic flux confined in an integrated magnetic core to the locations where magnetic actuation or sensing take place.
In further regard to the device disclosed in Ahn et al. are the structural configuration and fabrication technique utilized therein. Structurally, the rotor is spaced from the substrate base and positioned in the same plane as the top layer of the core. The rotor rests upon a pin bearing surface disposed on the pin support. In fabricating this micromotor, the rotor is first formed on a substrate separate from the pin and stator. The rotor is then released from the substrate and microassembled onto the pin. With this hybrid-assembled rotor, the micromotor achieves almost the smallest contact gap possible between the pin and rotor. Even though such optimal contact gap is achievable through the teachings of Ahn et al., the difficulties associated with microassembling the rotor onto the pin were not resolved. Thus, it would not be feasible to mass produce magnetic micromotors economically because of high production cost and manufacturing difficulties encountered with hybrid-assembling techniques.
Thus, it can be seen that it would be desirable to fully integrate the rotor and pin in manufacturing, eliminating the need to microassemble the rotor onto the pin. However, one of the difficulties in fabricating a fully integrated magnetic micromotor is how to produce a rotor of precise dimensions and having uniform gaps between the rotor poles and stator poles. The photolithography techniques used in forming fully integrated electrostatic micromotors are not directly applicable to magnetic micromotors for two reasons. First, electrostatic micromotors are thin film structures and magnetic micromotors are thick film structures. Secondly, electrostatic micromotors are planar structures with essentially all components in the same plane. In contrast, magnetic micromotors require multiple layers for construction of the multilevel wrapped coil. Therefore, to date, no known fully integrated magnetic micromotors have been developed.